1. Technical Field
The present invention relates to an asymmetric power divider.
2. Description of the Related Art
As the paradigm of mobile communication systems was changed from analog communication to digital communication and the demand for not only voice but also a large amount of data communication such as multimedia data was increased, a third generation communication based on digital communication appeared.
Due to the influence of communications requiring a large amount of data, a technology such as Orthogonal Frequency Division Multiplexing (OFDM) was adopted. Although this technology enables the number of accessing users to be further increased, it has a problem in that an amplifier having excellent linearity must be used because the difference between the average output and the maximum output is large.
In general, a linear amplifier is used for a communication system using the magnitude and phase of a signal, and has a large Peak-to-Average Power Ratio (PAPR) in the case of a large amount of data communication using a technology such as OFDM.
As a result, a linear amplifier does not need to be used in a maximum output region having the highest efficiency, but must be used in a region having back-off corresponding to PAPR.
Furthermore, Code Division Multiple Access (CDMA) and Wideband Code Division Multiple Access (WCDMA) systems which do not use OFDM have the same problem.
Meanwhile, although from the viewpoint of a probability distribution function, a region having the greatest transmission frequency is not a maximum output region but an intermediate output region, a problem arises in that the total average efficiency is low because the efficiency of the linear amplifier is low in the intermediate output region.
Accordingly, in order to overcome the problem of the low efficiency of a typical linear amplifier, a Doherty amplifier, such as that shown in FIG. 1 was proposed.
FIG. 1 is a diagram showing a conventional Doherty amplifier.
As shown in FIG. 1, the conventional Doherty amplifier employs a quadrature hybrid coupler 110 for producing input signals so that they have the same magnitude and a phase difference of 90 degrees and a λ/4 transmission line 140 for producing transmitted signals having a phase difference of 90 degrees for input and output. When the magnitude of an input signal is low, only a carrier amplifier 120 operates and the peaking amplifier 130 is turned off. As an input signal is increased, the peaking amplifier 130 issues an output along with the carrier amplifier 120. Accordingly, the conventional Doherty amplifier has the advantage of improving efficiency at intermediate output.
However, the conventional Doherty amplifier is problematic in that integration is difficult and loss due to the use of passive elements occurs because the sizes of passive elements used for input and output are large.
Furthermore, according to the conventional Doherty amplifier, although signals having a phase difference of 90 degrees, which is divided into halves by the quadrature hybrid coupler 110 are applied to the carrier amplifier 120 and the peaking amplifier 130 in the same manner, the peaking amplifier 130 is normally operated at a bias equal to or lower than a threshold voltage, such as class C, to perform the Doherty operation, so that the peaking amplifier 130 is not saturated as completely as the carrier amplifier 120 due to the signals applied in the same manner.
Accordingly, there arises a problem in that it is difficult to achieve desired output because the demodulation of the load R is not completely performed.
In order to solve these problems, an attempt to transmit more output to the peaking amplifier 130 by making the carrier amplifier 120 and the peaking amplifier 130 different in size has been made. However, this attempt cannot fundamentally overcome imbalance occurring at the input side.